The success of joint replacement surgery is primarily dependent on two factors, namely, (1) the position of the implant components with respect to the boney anatomy; and (2) the postoperative state of the surrounding soft tissues. Joints are surrounded by ligamenteous and capsular tissue. The state of these soft tissues determines the laxity and stability of the joint. The state of the soft tissues is effected by the following two factors: (1) the position of the components; and (2) the amount of soft tissue releases performed by the surgeon, as discusses in U.S. patent application publication No. 2005/0267485, which is hereby incorporated by reference in its entirety. In addition, a number of devices and techniques have been described that attempt to facilitate ligament balancing during a TKA procedure and in particular, some of these techniques involve trial prosthesis components which are used after femoral and tibial bone cuts are made to assess ligament tissue. See, U.S. Pat. No. 5,733,292, which is hereby incorporated by reference in its entirety.
Other devices are used to measure a gap between the distal femur and proximal tibia in extension and to help a surgeon recreate that same gap when the distal femur and proximal tibia in extension to help a surgeon recreate that same gap when the knee is in flexion. See, U.S. patent application publication No. 2003/0187452 and U.S. Pat. No. 6,575,980, both of which describe “gap checking” devices, each of which is hereby incorporated by reference in its entirety. Other devices have been developed to help measure an amount of ligament tension or to apply a desired amount of tension to the ligaments and in addition, paddle-like devices have been suggested for applying or measuring tension across a knee joint. See, U.S. Pat. Nos. 4,501,266; 5,597,379; 5,540,696; 5,800,438; 5,860,980; 5,911,723; and 6,022,377, each of which is hereby incorporated by reference in its entirety.
The device described in the above mentioned '485 publication is designed to determine the rotational alignment of the femoral component such that the knee is in optional tension; however, there are a number of disadvantages and limitations associated with this device. For example, the following are disadvantages associated with this device: (1) the distal femoral cut must be made first before the device is inserted and therefore, one can not change the planning in varus/valgus; proximal/distal, and flexion/extension; (2) the device must be fixed to the femur and therefore, requires bone screws on the medial and lateral sides which add invasiveness to the bone, as well as the soft tissues since access is required to the lateral side of the joint; (3) a vast number of components sizes for the tibia and the femur are still required; and (4) the system has constraints due to it not being able to account for different prosthetic designs using the same components, for example, different degrees of constraint or concavities of the tibial or femoral components.